Technical Field
Embodiments of the subject matter disclosed herein generally relate to preventing deposits (bio-fouling) on equipment (e.g., streamers, head buoys, tail buoys, etc.) used underwater for marine seismic surveys, and, more particularly, to pre-cut skin sections configured to cover and thus, protect portions of the equipment from fouling.
Discussion of the Background
In view of continually increasing interest in offshore drilling, marine seismic surveys are used to generate a profile (image) of the geophysical structure under the seafloor, based on which the presence or absence of oil and/or gas can be predicted. Marine seismic exploration surveys use seismic receivers such as hydrophones distributed along cables commonly referred to as “streamers” or “seismic streamers.” Plural seismic streamers and one or more seismic wave sources are towed together with other equipment (birds, buoys, deflectors, etc.). After a seismic wave source fires a shot (i.e., generates seismic waves), the seismic receivers acquire data related to reflections and refractions of the generated seismic waves. For example, the time interval between a moment when a shot is fired and when seismic receivers detect a shot-related reflection provides information about the location (depth) of reflecting interfaces inside the geophysical structure under the seafloor.
Seismic exploration campaigns may last long periods (e.g., several months). During these campaigns, the streamers and other equipment preferably remain in the water (except, for example, in cases of extreme unfavorable weather conditions) because recovering/deploying the seismic equipment is expensive and tedious. Thus, the streamers are immersed in seawater for several consecutive months, at a shallow depth (a few meters), and are generally towed at low speed (e.g., about 5 knots). In these circumstances, the streamers' outer surfaces are subject to fouling, particularly due to the proliferation of microorganisms (i.e., bio-fouling). One of the most common types of microorganisms attaching to streamers is barnacles which adhere permanently to a hard substrate either by growing their shells directly onto the substrate or by means of a stalk.
In time, bio-fouling results in substantial disadvantages such as:    (1) generating hydrodynamic flow noise;    (2) amalgams or incrustations forming on the streamer's outer surfaces, which are likely to disturb seismic measurements;    (3) increasing drag on the streamers and, consequently, increasing fuel consumption to tow them;    (4) a strong, unpleasant stench developing within a few days when streamers covered with bio-fouling are recovered and exposed to air; and    (5) streamer skin being pierced by certain types of bio-fouling, resulting in liquid intake inside the streamer.
Several techniques have been applied conventionally to address the bio-fouling problem. For example, a cleaning device with rotary brushes and/or blades may be temporarily or permanently attached on submerged streamers to clean their outer surfaces. The use of cleaning devices is impeded by the presence of protruding elements (e.g., floaters, or trajectory correction devices known as birds) on the streamers. Although cleaning devices continue to be subject to research and improvement, their practical value (efficiency versus cost) remains questionable.
Another conventional technique used to address the bio-fouling problem involves using antifouling paints (also used on boat hulls). However, antifouling paints do not adhere satisfactory to the polyurethane outer layer of the streamers and therefore do not resist prolonged exposure while streamer is deployed and towed. Furthermore, techniques for applying antifouling paints are hardly compatible with technical and economical constraints linked to seismic streamers because applying these paints is time consuming and costly operation due to the streamer's length. In addition, in order to allow the paint to dry, the painted seismic streamers have to be stored in a manner that requires a considerable amount of space, which is not economically viable onboard a vessel.
Applying a copper coating on the streamer's skin and loading the streamer's skin with a biocide substance are other techniques that have been considered for addressing the bio-fouling problem. However, the copper coating and the streamer's skin are not removable, so the streamers still have to be cleaned in tedious and expensive ways.
Accordingly, it would be desirable to provide devices, systems and methods addressing the bio-fouling problem related to streamers and other equipment used for marine seismic surveys, in an economically attractive manner and avoiding the afore-described problems and drawbacks.